The present invention relates to an apparatus and a method for producing optical data carriers.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
Optical data carriers for storing information have become ubiquitous in our daily lives during the past years. In particular, CDs (compact disks) and DVDs (digital versatile disks) have become mass storage devices. Unlike data storage devices with short access times, which are used particularly for random-access memory of computers and the like, optical data carriers are used for mass data storage, in particular for dealing with an ever increasing amount of data.
Practical applications of optical data carriers go back to the year 1980 when the so-called “laser disk” was developed. Subsequently, in the years 1982 the audio CD was developed, in 1985 the CD-ROM, in 1988 the MO-disk (magneto-optical disk), in 1989 the CD-R, in 1994/1995 the DVD, in 1996 the CD-RW, DVD-video and DVD-ROM, in 1997-2002 the DVD audio, the DVD-R, the DVD-RAM, the DVD-RW, the DVD+RW, and the DVD+R.
Another milestone has been achieved with the final stages of the development of the so-called BluRay disk. The development of the BluRay disk was concluded in the year 2003. BluRay disk acquires its name from the employed laser which has a wavelength of 405 nm, which is in the blue spectral range. An alternatives to the BluRay disk is, for example, the “high-capacity” DVD, also referred to as blue DVD.
Audio CDs were capable of storing at most 80 minutes of music information at a sampling frequency of 44.1 kHz and with a resolution of 16 bits. The CD-ROM achieved a storage capacity of about 700 MB. The smaller structures of the DVD enabled a significant increase in the storage capacity. For example, a so-called single-side, single-layer DVD (DVD-5) stores about 4.7 GB. A double-sided, double-layer DVD attains about 17 GB.
Both CDs and the DVDs store information in a spiral groove in the form of so-called “pits” (holes) or “grooves” (for recordable optical data carriers) and “lands” (bottom region). A “pit” is referred to as a hole in relation to the reflecting layer. An information layer having “pits/grooves” and “lands” is formed so as to be fully or partially reflecting. The hole (pit) is a hereby weakly reflecting. “Land” is referred to as the area between two “pits” and is strongly reflecting. Conversely, a “bump” refers to a “pit” as viewed by the laser.
The basic operation of scanning (reading) the information on a conventional optical data carrier (CD/DVD) will be described with reference to FIG. 1. A laser diode 10 emits laser light at a certain wavelength (CD: 780 nm; DVD: 650 nm; BluRay: 405 nm). The laser light is directed by a lens 12 to a polarizing beam splitter 14, from where it propagates to a so-called λ/4 plate which rotates the polarization of the laser beam. A lens 18 focuses the beam onto the track of an optical data carrier 20. Depending if a “pit” or a “land” is encountered, the laser beam is reflected with more (land) or less (pit) intensity. The reflected laser beam is again directed through the lens 18 to the λ/4 plate where the polarization is once more rotated. By rotating the polarization, the beam splitter 14 prevents feedback to the laser diode and directs the reflected laser beam through a lens 22 onto a photodiode 24 which determines if the laser beam is retro-reflected or not. A corresponding electric signal is generated by the photodiode depending on the information on the optical data carrier 20.
The overall dimensions of a conventional CD and DVD are typically identical. As shown in FIG. 2, both optical data carriers 20 have a diameter of 120 mm, a central bore 30 of 15 mm, and an annular data region 26, wherein the width of the annular region is approximately 30 mm. However, the technical data for the CD and the DVD are different. For example, the track spacing on a CD is 1.6 μm, the pit width 0.5 μm, and the maximum pit length 3.05 μm. The reading speed of a CD is 1.2-1.4 m/s.
A DVD (e.g., single-layer, single-side DVD) has a track spacing of 0.74 μm, a pit width of 0.32 μm, a minimum pit length of 0.4 μm and a maximum pit length of 1.87 μm. The reading speeds is approximately 3.49 m/s. Additional data for the numerical aperture (NA) and the thickness (T) through which the laser beam passes are indicated in FIG. 7.
FIG. 8 shows once more on an enlarged scale the different configuration of a CD (left image) and a BluRay (BR) (right image). A CD is essentially formed of a substrate 17, with the information layer (sequence of “pits” and “lands”) formed on the bottom side of the substrate 70. This side is coated with a reflecting layer 72. Thereafter, a thin protective layer, mostly in form of a clear varnish 74, is applied. A label is then typically applied on top of the clear varnish 74. The light passes through the CD from above through the substrate 70, which has a thickness of approximately 1.1 mm. The numerical aperture is 0.45. A scratch on the surface of the CD does not significantly affect the light signal.
For a BluRay disk, the cover layer through which the light passes has a thickness of only 0.1 mm. This layer is followed by the reflecting layer 72″ and a support layer 74″ underneath. In a BluRay disk, the light passes only through the thin upper layer of 0.1 mm, with a numerical aperture of 0.85. A scratch in the surface can adversely affect the signal quality.
FIG. 6 illustrates the difference between a DVD-5 and a BluRay. Both optical data carriers have a substrate layer 50, which for the DVD (left image) is in a range of 0.6 mm and for the BluRay in a range of about 1.1 mm. This layer 50 is followed by the information layer 52 in which the “pits” are arranged. Thereafter (in FIG. 6 below), a transparent cover and protective layer 54 is provided which for the DVD has a thickness in a range of 0.6 mm, for the BluRay however only in a range of 0.1 mm. The reference symbol 56 indicates the minimum pitch length of 0.4 μm for the DVD, the reference symbol 58 the track spacing of 0.74 μm. The reference symbol 60 indicates the minimum pit length of 0.15 μm for the BluRay, the reference symbols 62 the track spacing of 0.32 mm. Another partial cross-sectional view of a DVD is shown in FIG. 3.
Additional data formats for a DVD are illustrated in FIGS. 5a to 5d. The schematic diagram of FIG. 5a illustrates a single-layer, single-sided DVD, the diagram of FIG. 5b a single-sided, double-layer DVD, the diagram of FIG. 5c a double-sided, single-layer DVD, and the diagram of FIG. 5d a double-layer, double-sided DVD. In this context, the term “single-sided” indicates that information is applied to only one side of the DVD and can only be read from one side. The other side is a dummy side. Double-sided indicates that the information is stored on both sides of a DVD and must also be read from both sides. When playing the DVD, the DVD must be flipped over to read the information on the other side.
Single-layer indicates that information is provided on the layer on one side of a DVD (general optical data carriers). Double-layer indicates that two information layers are arranged on one side of a DVD (universal optical data carriers). In this case, the upper information layer is formed as a semi-transparent layer, so that a suitably focused laser directed onto that layer can be steered to the lower information layer and is able to also read the information from the lower layer.
To date, CDs have been produced by an injection molding process or an injection compression process. First, a die is arranged in a cavity of a mold of an injection molding machine, with the information to be transferred to the CD already provided on the die. The structures of the die are impressed onto the CD blank when the plastic material is injected into the cavity, thereby transferring the information from the die to the plastic material. The side having the structure/information is subsequently coated with a reflecting layer and varnished in a following step. The thickness of the plastic part forming the substrate is in a range of 1.1 mm for a CD so that the pit structures could be easily formed using an injection molding process. Corresponding manufacturing facilities have operated reliably throughout the years.
However, significant problems were encountered in the production of a DVD due to the smaller pit size. Moreover, the cover layer through which the laser beam passes must be made thinner, which depends in particular on the particular wavelength of the laser light and the numerical aperture. With a CD, an infrared laser with a wavelength of 780 nm is typically used, whereas a DVD employs a wavelength of about 635 to 650 nm. As a result, with a final thickness of the optical data carrier again in a range of 1.2 mm, two separate disks each having a thickness of approximately 0.6 mm had to be produced, which then had to be glued together. Manufacturing problems were already encountered in the production of the relatively thin disks of 0.6 mm because, for example, the pit geometries could not be imaged with the required accuracy when using thermoplastic material that has a relatively high viscosity in the molten state. The gluing process also has been found to be susceptive to failure. In the first years of DVD manufacturing, approximately 50% or more of the DVDs had to be discarded, because bubbles where found in the adhesive layer. Even today, the rejection rate of DVDs is in the range of 15-20%.
Additional problems were encountered in the application of the reflecting layer and/or the partially reflecting layer. The reflecting layer should faithfully reproduce the underlying pit-land structure. Depending on the coating process (e.g., with the so-called sputtering process), the coating structures were found to no longer identically reproduce the underlying pit-land structure.
The above general discussion is intended to provide a better understanding for the problems encountered in the production of the BluRay disks. With this novel optical storage medium, the transparent cover layer through which the light passes can have a thickness in the range of only 0.1 mm. In addition, the track spacing—as mentioned above—is reduced to 0.3 μm and the minimum pit length is in the range of 0.16-0.138 μm. These differences were already discussed with reference to FIGS. 7 to 9. A layer thickness of 0.1 mm is difficult to achieve using an injection molding process. Initial attempts of applying a cover layer therefore employed a spin-coating process. In this approach, a certain quantity of material for the cover layer is applied, and the data carrier is rapidly rotated, thereby uniformly distributing the material of the cover layer toward the outside by centrifugal force. However, it has been observed that the layer thickness is not completely uniform, and that a layer thickness of exactly 0.1 mm cannot be precisely attained. Additional experiments were carried out with adhesive foils; however, these experiments did not give satisfactory results.
It would therefore be desirable to provide an apparatus and a method which is able to overcome the aforementioned shortcomings by providing optical data carriers having conventional CD and DVD formats, as well as optical data carriers with information and cover thin layers and small-size information structures.